The present invention relates to the measurement of the current density distribution in electron and ion beams, particularly to a modified Faraday cup having radial slits therein to create an image of the current density of such beams, and more particularly to an enhanced modified Faraday cup utilizing two spaced slit disks, one disk having one slit wider than the other slits, and a ring to help minimize the amount of secondary electrons and ions from being produced.
Electron beams are considered to be the most precise and clean method available for welding thick sections of materials. Unfortunately, electron beams suffer one critical deficiency, namely the repeatability of focusing the beam to a known power density. Without the ability to reliably reproduce the power distribution in an electron beam, weld quality cannot be guaranteed. This problem is exacerbated by the fact the many welds are made over a period of time and with different welding operators. Further complications arise when welds are developed on one machine than transferred to a different machine for production. An electron beam diagnostic method has been developed that enables the precise characterization of the power density distribution in high power electron beams. Such diagnostic method, which utilizes a modified Faraday cup, is exemplified by U.S. Pat. No. 5,382,895, U.S. Pat. No. 5,468,966, U.S. Pat. No. 5,554,926 and U.S. Pat. No. 5,583,427. This electron beam diagnostic method has been utilized, for example, to certify changes in electron beam welders, and is further described in J. W. Elmer et al, "Tomographic Imaging of Non-Circular and Irregular Electron Beam Power Density Distributions," Welding Journal 72 (ii), p. 493-s, 1993; A. T. Teruya et al, "A System for the Tomographic Determination of the Power Distribution in Electron Beams", The Laser and Electron Beam in Welding, Cutting, and Surface Treatment State-of-the-Art 1991, Bakish Materials Corp., p. 125, 1991; and J. W. Elmer et al, "Beam Profile Analysis for the C&MS B231 Electron Beam Welding Machines", LLNL UCRL-ID-127549, Jun. 12, 1997.
The present invention provides an enhancement of the modified Faraday cup (MFC) diagnostic device utilized in the above-referenced patents, and specifically provides an improvement over the MFC of above-referenced U.S. Pat. No. 5,583,427. The enhanced MFC of the present invention improves the quality of the signal that is measured by the MFC, and thus improves the accuracy of the power density distribution measurements. In the MFC of U.S. Pat. No. 5,583,427, the electron beam is oscillated around a tungsten slit disk which samples the beam. The sampled beam current is then measured with an MFC. The MFC of the patent suffers from two problems. First, a substantial percentage of the electron current passing into the Faraday cup could be transported as secondary electrons and/or ions back up to the tungsten slit disk, and therefore would not be properly accounted for. Second, with repeated use, the electrical contact between the tungsten slit disk and the copper heat sink body would degrade. Also, when measuring non-circular beams with the prior MFC, there was no method to orient the measured beam profile with respect to the welding chamber.
The present invention overcomes the above-mentioned electron capture problems by the inclusion of several significant additions to the MFC, of which includes a second slit disk located inside the Faraday cup, a ring added in the Faraday cup below the second slit disk, a beam trap added within the Faraday cup, an improved ground arrangement for the tungsten slit disk, and modifying the tungsten slit disk to orient the beam profile with respect to the welding chamber.